I have a history of depression and anxiety, and I always erred on the safe side by not taking anything while pregnant or breastfeeding. This worked out OK for me, but I know some people need the medication.
I would not rely on your doctor's willingness to prescribe it alone as proof of its safety. My doctor always gave me the risks/benefits and made it my choice. I would read the actual drug information on the clinical trials and the adverse effects found in breastfeeding infants. Often, no adverse effects were found, but these things can crop up in the years to come.
It is a hard call. Personally, I would not take anything unless it has been proven to be completely safe for nursing mothers. If I felt I needed a drug that had not been proven safe or if the effects were unknown, I would probably stop breastfeeding. It is a matter of really weighing out the risks and benefits to figure out which option is best for you and your child. Good luck.